This invention relates to a system for securing end fittings for light weight, but strong composite tubing and the like. The system is particularly useful with tubing subjected to high operating pressures, one example of which is use in aircraft hydraulic systems.
Composite tubing has proven to be increasingly useful in many applications due to its high strength and low weight. A prime example of this is the use of composite tubing in aircraft high pressure hydraulic systems. Improvements in materials has created composite tubing that can provide the necessary strength but with less weight than that of metal. As is well known, any weight saving is of great value in aircraft because of energy conservation and resulting cost saving.
Such composite tubing typically is being made of large numbers of strong, long fibers or filaments bonded together by suitable resins to create a strong rigid matrix. Various materials are being used for fibers, such as Kevlar, carbon and glass. The fibers are arranged in a wide variety of patterns, usually involving many layers of crossed fibers. For example, one or more layers may be arranged spirally around a mandrel alternately arranged with other layers of fibers spiraling in the opposite direction so that the fibers intersect. In other approaches, the fibers are braided or woven, still resulting in crossed fibers or mesh-type constructions. Resin is applied to the fibrous mesh in various ways or stages. In high pressure fluid applications, the matrix has a thin inner liner of metal or other fluid impervious material to provide additional sealing capability.
Currently, end fittings for composite tubing are attached by winding filaments over one or more ridges on the end fitting and curing the composite so that the ridges support the longitudinal loading, by attaching end fittings with adhesive, or by mechanically squeezing the tubing wall between ridged, tapered and/or grooved mating sections that grip the ID and OD of the tubing. Curing the end fitting in place requires that each tube length be fabricated to size at the factory. Fitting ends attached with adhesive do not provide the strength for high pressure applications. Current mechanical means do not provide sealing for high pressure applications.
Also, there is a need to fabricate tubing lengths for repair of damaged tube assemblies in the field. This can be for permanent installations like oil field pumping stations, for aircraft at remote air fields or carriers, or for space stations in orbit.
The problem of damaged tubing has sometimes been handled by shipping the tubing back to the factory where end fittings can be installed. Because it is not practical to let expensive equipment sit idle, a damaged tube has instead typically been replaced by a new one, while the damaged one if repairable was returned to the factory. This in turn creates a need for carrying inventory of adequate replacement tubes. Compounding this problem is the fact that most tubing in aircraft hydraulic systems is cut to a precise length. Thus, a supply of a few sizes is not sufficient, but instead it is necessary to have a supply of each particular tubing that has a likelihood of being needed for replacement.
While there are many different systems shown in the prior art for connecting end fittings to a tubular member, the high pressure operation must be kept in mind. For example, commercial airplanes typically have hydraulic systems operating at 3000 psi; military planes typically have operating pressures of 4000 psi and some newer ones reaching 5000 psi. Moreover, there are some research and development activities utilizing pressures in the range of 8000 psi. In addition, it is necessary from a safety standpoint that such tubing be able to withstand three to four times the operating pressures. By extreme contrast, everyday garden hoses are typically subjected to no more than a 100 psi.
Metal tubing, which is replaced by the composite tubing, may have a circumferential score line formed in its periphery to assist in connecting an end fitting, but such an approach is unsatisfactory for composite tubing because the fibers cannot be cut without having an unacceptable corresponding loss of strength.
Thus, there is a significant need for an improved end fitting for composite members, particularly tubing subjected to high operating pressures such as in hydraulic systems. Such a fitting should be capable of easy connection on the job site without the use of complex machinery so that a supply of uncut tubing may be kept at the job site and then cut to a desired length for connection to an end fitting. A satisfactory fitting can of course not be significantly detrimental to the resin fiber matrix of the tubing.